The purpose of this study is to investigate the possibility of association between prenatal or early infantile exposure to measles, mumps, chicken pox and rubella and the development of autism. These viral diseases have been selected for study because they are known to cause encephalitis and theoretically may be vehicles of chronic neurologic damage. Autism is selected because many features suggest an organic basis. Two groups of index cases will be assembled. One will consist of approximately 100 children with the fully developed syndrome of autism. The other will be comprised of children with some, but not all, of the features of autism. Histories of clinical illness with the four viral diseases during the mother's pregnancy or in the first 18 months of the children's life, or of exposure at these times to clinical illness in a sibling, will be obtained from parental interviews and obstetric and pediatric records. The expectation of infection during pregnancy (in the absence of a causal association) is essentially zero in series of this size. On other kinds of exposure, comparison will be with frequency of the comparable exposure in the index cases' siblings.